Field of the Invention
The present invention relates to a robot apparatus provided with a robot arm that has a plurality of sensors of which the detectable ranges for a force are different from each other; a method for controlling a robot; a program; and a recording medium.
Description of the Related Art
Along with a development of a robot technology, such a tendency is growing as to use a robot apparatus which operates automatically according to a control program, for production use of articles. Among the above articles, as regarding the production of articles for which an assembling operation is required, a robot apparatus in which a force can be controlled is progressively introduced. For instance, a robot apparatus is known which has a sensor such as a force sensor arranged therein, for finding a force exerted on an end portion of a robot arm (occasionally referred to as force sensor in the present specification). In addition, a robot apparatus is known which has a sensor such as a torque sensor arranged therein, for finding a force exerted on each joint of a robot arm (occasionally referred to as torque sensor in the present specification). These robot apparatuses can control the force (control an orientation) exerted on a robot containing its end-effector, when assembling parts by the robot, by sensing an external force generated in an end-effector at the time of an assembling work.
The robot apparatus which has a sensor such as a force sensor arranged in the end portion of the robot arm can arrange the sensor near the end-effector, and accordingly can sense the external force generated at the time of the assembling work, with little loss. By sensing the fine external force by this sensor, the robot apparatus enables precise force control.
The robot apparatus which has a sensor such as a torque sensor arranged in each joint of the robot arm can sense the external force, even when a contact object comes in contact with not only the end-effector but also anywhere of the whole robot, by arranging the sensor in each joint. On the other hand, there is a case where it is difficult for the robot apparatus to sense the fine external force with high precision and control the force precisely, due to losses such as friction in a driving portion of each joint, an influence of an inertia force of the robot arm itself, and the like.
In the future, along with a further progress of automated production, there is a possibility that a work range which is required to the robot arm increases, for instance, to such an extent that one robot arm assembles one product. For instance, in electrical products, the work range includes a wide range from an assembly of flexible parts such as a flexible cable, to an assembly of rigid-body parts of metal or the like. In the viewpoint of the force control, the robot apparatus results in being needed which copes with forces from the fine external force to a large external force.
Conventionally, a method has been proposed which determines an appropriate force control parameter according to a variation of a working object and performs force control of a robot (see Japanese Patent Application Laid-Open No. H02-139190). In this Japanese Patent Application Laid-Open No. H02-139190, it is described that parameters are changed for each working object, which include a force feedback gain and a gain of a compensator necessary for the force control, a filter constant, a width of a dead zone and a servo period.
However, a detectable range of a sensor is limited which senses the external force, and accordingly when a parameter is only changed for each working object of the above described force control robot, there have been the following problems.
(1) When the sensor is used which can sense a large external force, the sensing result of a fine external force is buried in noises, and there is a region in which the force cannot be controlled with high precision.
(2) When the sensor is used which can sense a fine external force, the sensing result of a large external force is saturated, and there is a region in which the force cannot be controlled with high precision.
Then, an object of the present invention is to sense forces from the fine external force to the large external force with high precision, and enable high-precision control.